1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to an alkali metal/solid cathode electrochemical cell having reduced voltage delay and irreversible Rdc growth. A preferred couple is a lithium/silver vanadium oxide (Li/SVO) cell. In such cells, it is desirable to reduce voltage delay and permanent or irreversible Rdc growth at about 25% to 70% depth-of-discharge (DOD) where these phenomena typically occur.
2. Prior Art
Voltage delay is a phenomenon typically exhibited in an alkali metal/solid cathode cell, such as of lithium/silver vanadium oxide couple, that has been depleted of about 25% to 70% of its capacity and is subjected to high current pulse discharge applications. It is theorized that vanadium compounds may become soluble in the cell electrolyte from the SVO cathode and be subsequently deposited onto the lithium surface of the anode. The resulting anode surface passivation film is electrically insulating, which leads to cell polarization.
The voltage response of a cell that does not exhibit voltage delay during the application of a short duration pulse or pulse train has distinct features. First, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse, and second, the minimum potential of the first pulse in a series of pulses is higher than the minimum potential of the last pulse.
On the other hand, the voltage response of a cell that exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied.
Thus, decreased discharge voltages and the existence of voltage delay are undesirable characteristics of an alkali metal/silver vanadium oxide cell subjected to current pulse discharge conditions in terms of their influence on devices such as implantable medical devices including pacemakers and automatic implantable cardiac defibrillators. Depressed discharge voltages and voltage delay are undesirable because they limit the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.
Therefore, there is a need for a lithium/silver vanadium oxide cell that is dischargeable to deliver the high capacity needed for powering implantable medical devices and the like, but that experiences little, if any, voltage delay and irreversible Rdc growth during pulse discharging, especially at about 25% to 70% DOD.